Gamma type cylinder and displacer

[czakun]

Hi all,
I'm preparing to make my first Stirling engine. It's gonna be gamma type. Cylinder and displacer gonna be made of some kind of metals instead of- popular in that purpose- glass test-tubes which are hard to find in suitable dimensions and are easy to damage. Got doubt about it.
What features should have materials cylinder and displacer are made of? Hot and cold part of cylinder should be made of well conductive
material? Which material use to separate these? Displacer from not conductive one? Am I right? What materials can you suggest for each of them? Availability and easy tooling are required.

[Ian S C]

For the displacer and hot cap, the best material to use is steel, preferably stainless steel. You want a metal that will not conduct heat rapidly from the hot to the cold end. the resistance to heat conduction directly into the hot end is not impeded as it only has perhaps .5 mm/the thickness of the hot cap to travel. Don't be tempted to use aluminium for the displacer (many do use it), (A) it conducts the heat too fast from hot to cold. (B) If you (as I do) use gas/LPG as fuel, the aluminium will collapse.

Ian S C
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[czakun]

Thanks for your reply ;) Didnt know the underpressure inside is big enough to collapse aluminium tube!
Another problem has occured. As there are only "both-side opened pipes" to purchase (should just say a "pipe"? ) how could I close it? With kind of metal sheet? How to do that?
EDIT How thick should be these pipes? Thick as possible? Wont collapse like alu?

[Ian S C]

For the hot cap, about .5mm in the area to be heated is a good thickness. The displacer could be as thin as .25 mm, I have made them as thin a .007"/.018 mm. Of latter years I have used thin walled tube with an end TIG welded on, I'v got a friend who is a commercial Stainless steel fabricator, he didn't like the .018 mm tube.
Before that the hot caps were machined out of solid 316 stainless bar. The open end has a fine thread, this screws on to the cold end and becomes a BETA type motor.
Ian S C

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[czakun]

Ok I will look for tubes as thin as possible. For now I've only found 1mm with diameter I want (about 20-24 mm). I guess its not possible to lathe it to 0,25-0,5mm? Which ones are better annealed or not annealed?
I've found video with another approach where autor is soldering the end of cylinder (maybe it could be helpful to someone).
https://youtu.be/CmLtUkplsfc?t=116

[Ian S C]

If the tube is smooth on the inside, without the ridge caused by welding, a mandrel can be pushed through, and the outside turned down to the desired thickness. I think you should be ok with the 1 mm for the hot cap, but the displacer should be as light as possible. As long as you are not going to run the motor at red heat, you could braze the end on.
When things get hot, aluminium looses all its strength, and my motors are designed (?) to run at red heat.
Ian S C

[czakun]

Thanks Ian, you are very helpful

[czakun]

Another problem occured. Which is better: long displacer cylinder with small diamater or short with bigger diameter? Both with same volume

[Ian S C]

The normal design of a displacer is that the length should be three times the diameter, although this is not always the case, the Robinson type motor has a shorter displacer, about 1 : 1, but has a internal regenerator. With the 3 : 1 displacer the length of the displacer acts as a regenerator, you can see where the heat has been on stripping the motor, the hot end is blue for about a quarter of the length, gradually fading toward the cold end.
Ian S C

[czakun]

OK, lets say there is a gamma Stirling engine with 20x60mm displacer. The swept volume of displacer cylinder should be 1.5 of swept volume of piston cylinder. Thats theory.
1) These considerations dont define these volumes, just the ratio between these. What about displacer stroke and its cylinder volume?
2) What about a part connecting both of cylinders? Its volume should be as small as possible? Where should it be placed? Closer to hot or cold end of displacer cylinder?
3) What about wokring pistons diameter to stroke ratio?

[Ian S C]

I can only go by what I build, I make the exit from displacer cylinder to power cylinder right at the cold end (I have seen plans where the exit is at halfway). Don't make the transfer port too small. On my motors I try to make the piston larger than the stroke, maybe I'm wrong, but I think the larger the area of the top of the piston the better, it's pounds per square inch (or what ever) that powers the motor, and there is very little of that. Going by IC theory, short stroke, high revs. But aim for as long, within reason connecting rod to reduce side pressure of the piston against the cylinder wall. You can either have both cranks of equal stroke, and the power piston smaller in dia than the displacer, or both the same size, and the displacer with a longer stroke.

Ian S C

[czakun]

And what do you think about displacer volume to its cylinder volume? I mean what about displacer stroke when you got displacer with given, particular dimensions?

[czakun]

But aim for as long, within reason connecting rod to reduce side pressure of the piston against the cylinder wall.

I'm not sure what exactly do you mean...
Can you please tell what about displacer cylinder smoothness? Should it be rough to maximize temperature exchange surface?
Can someone please answer my previous post? I mean how long should be displacer cylinder compare to displacer length?

[Ian S C]

The displacer cylinder should be long enough to give a small ( 1 mm or so) gap at each end, so the stroke plus about two mm is about what I work to. A reasonable as turned finish for the displacer and hot cap.
If the con rod is short it will form a large angle tending to push the piston sideways, creating friction, while this is not a great problem in the average IC motor, any friction you can eliminate is good for Stirling Engines.
Ian S C

[czakun]

Thanks Ian for keeping be very helpful
I know there should be a gap at both ends. What were I asking was about proportions of displacer length to its stroke.
I have ordered tubes today. Unfortunately I couldnt find seamless, so I bought welded ones with dimensions 26,9x1,6mm and 25x1,5mm. Will see if i can machine them to work well together...